RAIM is the abbreviation for Receiver Autonomous Integrity Monitoring, a technology developed to assess the integrity of Global Positioning System (GPS) signals in a GPS receiver system. It is of special importance in safety-critical GPS applications, such as in aviation or marine navigation.
RAIM detects faults by utilizing redundant GPS pseudorange measurements. That is, when more satellites are available than needed to produce a position fix, the extra pseudoranges should all be consistent with the computed position. A pseudorange that differs significantly from the expected value (i.e., an outlier) may indicate a fault of the associated satellite or another signal integrity problem (e.g., ionospheric dispersion). Traditional RAIM uses fault detection only (FD); however, newer GPS receivers incorporate Fault Detection and Exclusion (FDE) which enables them to continue to operate in the presence of a GPS failure.
Because RAIM operates autonomously, that is, without the assistance of external signals, it requires redundant pseudorange measurements. To obtain a 3-dimensional position solution, at least 4 measurements are required. To enable RAIM FD (Fault detection in RAIM), at least 5 measurements are required, and to enable RAIM FDE (Fault detection in RAIM with the ability to exclude faulty data), at least 6 measurements are required. However, more measurements are often needed depending on the satellite geometry. Typically, there are 7 to 12 satellites in view.
Conventional RAIM availability thus requires 6 or more satellite measurements with good satellite geometry. This is two or more satellites than is required for the basic navigation solution. However, if time can be eliminated from the list of unknowns, and thus drop the required number of satellites from 4 to 3, then RAIM FDE can be achieved with only 5 satellites. Time can be eliminated by proving the GPS receiver with a precise time reference such as that available from an atomic clock.
Since GPS requires “line of sight” reception to receive the GPS navigational signal, terrain surrounding runways can occlude one or more of the satellites at critical times. Removal of one or more of the several satellites compromises or prevents the availability of RAIM. Aircraft GPS precision approaches are frequently interrupted by RAIM outages. Certain flight operations, such as precision approach, can no longer be executed without RAIM availability.
There is an unmet need in the art for improving the availability of RAIM by using the aid a precise and accurate time signal.